Conventional blind fasteners, such as blind rivets, comprise an outer tubular shell having an enlarged flange at one end, together with a mandrel associated therewith, such mandrel comprising a cylindrical stem extending through the tubular body so as to be co-axial therewith and the stem being coupled with a remote end of the body, usually by having a radially enlarged head at one end for engagement with an end-face (tail end) of the rivet body remote from the enlarged flange. The blind rivet is passed through a preformed hole in a workpiece until the flange engages with the edge of the hole and is held in engagement therewith during a setting operation. During setting, the remote end of the rivet, which is disposed inwardly of the workpiece (the blind side), is then compressed towards the flange by drawing the mandrel stem, and hence the mandrel head, back towards the flange, whereby the deformed portion of the rivet body compresses the workpiece therebetween with the flange. Conventionally, many mechanical assemblies use blind rivets to secure one or more components together in a permanent construction. Blind fasteners are preferred where the operator cannot see or access the blind side of the workpiece—for instance where the rivet is used to secure a secondary component to a hollow boxed section. Blind rivets are also preferred where a high volume of assemblies are being produced as there are advantages to be gained from increased assembly speeds and productivity compared with, say, threaded or bolted joints.
However, one of the disadvantages of a blind rivet setting is that the blind side of the set rivet is often inaccessible and therefore cannot be visually inspected to determine a correctly completed joint. Additionally, even if visual inspection was possible, for rivet setting operations utilising a plurality of blind rivets of different sizes for insertion in different sized holes, a visual inspection may also fail to identify if an incorrectly sized rivet has been used in a particular hole diameter. Alternatively, in automated blind rivet setting procedures it is also possible that a blind rivet may not be set at all during a particular automated cycle or may be “free set” in air remote from any workpiece. A secondary operation of visually or manually inspecting an assembly following a preset automated blind rivet setting operation would introduce an additional manufacturing step and associated expense into the manufacturing procedure.
To address such problems, automated blind rivet setting monitoring operations have been developed which effectively measure the force applied to the rivet mandrel during a cyclic fastener setting operation. For example, the applicant's earlier European Patent No. EP 738 551 measures the load applied to the mandrel stem during the rivet setting operation against the displacement of the piston assembly within the rivet setting tool, and analysing the results of such measurements against pre-determined settings to determine whether the set rivet falls within acceptable parameters and can be considered as a “good” set. This disclosure further discusses the benefits of analysing the velocity of the piston displacement compared to the applied load to also compare against pre-determined values.
A second patent in the name of the Applicant, EP 738 550 discloses a similar means of analysing the setting operation of a blind rivet but in this case measures the setting force against displacement of a gripping mechanism of the rivet setting tool so as to analyse the total energy employed during the rivet setting operation, and to compare with pre-determined values to determine whether or not the set blind rivet falls within acceptable parameters.
Whilst both the aforementioned analysis techniques provide a very thorough and effective means of determining the quality of a set blind rivet, both employ complex analytical techniques in order to determine the quality of the setting operation, usually by monitoring step by step, almost continuously, the resulting load/pressure-displacement curve, requiring complex software to effect such analysis adding considerable cost to the rivet setting equipment. Additionally, since the analysis techniques are relatively complex, such techniques do not lend themselves to a high degree of flexibility in readily adapting the apparatus to analyse the setting operation for different types and sizes of blind rivet, particularly where different fasteners are used sequentially.
A more simplified rivet setting monitoring process is also disclosed in German Patent Specification DE4217901 (Honsel) which simply measures the displacement force exerted by the setting tool against the displacement of the piston of the setting tool, and from analysis of such results determining if a set rivet is within acceptable parameters. However, the drawbacks of all existing blind rivet monitoring processes is the necessity to use at least two transducers to not only measure the force applied to the rivet during the setting operation but to also measure a manual displacement of at least one piston of the rivet setting apparatus. In addition, none of the prior art has addressed the possibility of adapting such monitoring equipment to deal with large scale rivet setting operations utilising a plurality of rivets and/or rivets of different size and shapes. Prior art devices are limited to analysis of one type of blind rivet only at any one time.
Despite the various complex and expensive blind fastener monitoring systems currently available, a need has been further identified to provide a simplistic and inexpensive device and procedure to monitor the operation of a blind fastener setting tool in order to identify, and specifically provide an appropriate visual or audible warning of, the occurrence of a “free set” fastener setting operation in which a rivet may be set remote from the workpiece.
It is therefore an object of the present invention to provide a simplified method of monitoring the setting operation of such blind fasteners and a blind fastener setting system employing such method which alleviates the aforementioned problems in a cost effective manner and which has greater flexibility in its application to automated fastener setting operations.
According to the present invention there is provided a method of monitoring the setting operation for a blind fastener, comprising the steps of measuring, as a function of time, an electronic signal indicative of the load being applied to the fastener, and more specifically the load applied to the mandrel, during the setting operation, from such signal determining a mandrel entry load and an associated mandrel entry time; further determining mandrel break point or a setting load (mandrel break point load) and an associated mandrel break point time or setting time; subsequently, determining the time difference between the mandrel entry time and the mandrel break point/setting time and comparing this time difference against a pre-determined reference time difference value associated with the fastener to determine whether the set fastener complies with a pre-determined acceptable setting procedure.
Preferably, the method will also determine the difference between the mandrel break point or setting load and the mandrel entry load and compare this difference in load against a pre-determined reference load difference value to again determine whether this set fastener complies with pre-determined acceptable setting procedure.
In the event that the set fastener is determined to not comply with the pre-determined fastener setting procedure due to either or both of the load difference or the time difference being incompatible with the pre-determined difference values, then an output signal will be generated, which itself will either be audible or visual, so as to notify a user of a potential difficulty with the fastener setting procedure being monitored.
Preferably, this method will further comprise the step of analysing the difference between the determined time difference and the reference time difference when the fastener set is determined not to comply with the pre-determined fastener setting procedure, whereby such analysis will be used to identify the reason for non-compliance, usually by determining whether the difference is greater than or less than the pre-determined difference values which is indicative of certain known failure criteria.
In one preferred embodiment of the present invention, the predetermined reference time difference may be determined as the time difference between mandrel entry time and mandrel setting time of a blind fastener set in a known workpiece, and the step of comparing the measured time difference against the predetermined reference time difference comprising identifying whether or not the measured time difference is greater than the reference time difference by a predetermined value indicative of a free set operation and generating a reject signal in the event that such free set operation is detected.
Additionally, the method may further comprise the step of determining a minimum load value after the mandrel entry load is determined, which minimum load value having an associated minimum load time, subsequently comparing the at least one of the minimum load value or the minimum load time against a pre-determined minimum load value or pre-determined minimum load time to identify the reason for non compliance, again by determining whether the variation between the measured value and the pre-determined value is greater or less than, with such results indicative of certain known failure criteria.
Preferably, the method will further comprise the step of visually displaying a graphic plot of monitored load applied to the mandrel against time to aid visual interpretation of the setting procedure.
The method of the present invention is further applicable to a method of monitoring a series of setting operations for at least two different blind fasteners, comprising the step of pre-determining the sequence of blind fasteners to be set in the series and monitoring the setting operation of each of the fasteners in the series according to the method described above, wherein the pre-determined reference time and the predetermined reference mandrel load associated with each of the at least two different blind fasteners is pre-set against each of the setting operations for that particular fastener in the series. Specifically, the method employs the use of undertaking a series of monitoring procedures as previously described, whereby each monitoring procedure will be dependent on the pre-determined characteristics of the fastener being set which will be pre-set to an appropriate monitoring system. This specifically allows for the method to determine if the incorrect fastener is set out of sequence since its determined values will not comply with those pre-set for a different type of fastener.
Usually, the pre-determined reference load values associated with each of at least two different blind fasteners will also be pre-set against each of the setting operations for that fastener in the series.
Preferably, the pre-determined reference time is determined by undertaking a plurality of setting operations for the selected fastener type, preferably in the component being assembled, measuring a signal indicative of the load being applied to the fastener during the setting operation, as a function of time, from which signal measurements the mandrel entry load and associated mandrel entry time may be determined together with a setting load (mandrel break load) and associated mandrel break or setting times for each of the plurality of operations, following which the determined values of mandrel entry load, mandrel entry time, mandrel break or setting load and mandrel break or setting time for the plurality of operations are then averaged and from such averaged values the time difference between the average mandrel entry time and the average setting (mandrel break) time are calculated to provide this pre-determined reference time difference. Similarly, the pre-determined reference load can also be calculated by averaging the mandrel entry load and the mandrel break or setting load and determining the difference therebetween as the reference load value.
Alternatively, the pre-determined reference time may be determined by again undertaking a plurality of setting operations for each selected fastener type and for each of the plurality of operations, preferably in the component being assembled, determining the time difference between the mandrel entry time and the mandrel break or setting time and then simply averaging the determined values of these time differences for the plurality of operations to provide the pre-determined reference time. In both situations, the pre-determined values against which each subsequent operation is to be compared to determine whether or not the fastener setting procedure is acceptable, may be achieved through a self learning process and by measuring the operation and setting of the fasteners in situ and thus each pre-determined reference time or reference load can be calculated dependent on the exact situation in which the fasteners are to be employed. Again, the pre-determined reference load difference may alternatively be calculated by measuring the difference between the mandrel entry load and the (or mandrel break) setting load for each of said plurality of operations, and averaging these load differences to obtain a reference load value difference.
Usually, where the method is applicable to monitoring a series of setting operations, these multiple setting operations may be undertaken by a plurality of different setting tools wherein an electronic signal indicative of applied load to the mandrel is generated by each setting tool during a setting operation by that tool and each electronic signal is analysed sequentially according to the pre-determined order of setting of blind fasteners. Here the pre-programming of the series of setting operations not only allocates the order of fasteners to be set but also which setting tool is to set those fasteners and in which particular order, and which pre-determined values are to be applied to the monitoring operation for each setting operation.
The method as previously described, may be further used to determine wear on a set of jaws of a fastener setting tool by comparing the mandrel entry time against a pre-determined mandrel entry time. Here, if the fastener setting tool jaws are subject to wear then they may slip when engaging a mandrel stem of the fastener thus delaying the fastener setting cycle load being applied such that the mandrel entry load will be delayed to account for the slipping. This will allow the operator to monitor the performance of the components of the setting tool, but the effect of slippage will not affect the monitoring operation of the setting procedure itself since, once the mandrel is correctly gripped, such slippage will not affect the time between mandrel entry and mandrel setting.
Further according to the present invention there is also provided a blind fastener setting system comprising a fastener setting tool, a signal generating device for producing a signal indicative of the load being applied via the mandrel to a blind fastener during a setting operation, and a signal processor for measuring this signal as a function of time and performing the monitoring method for the setting operation as described above. Usually, the system may comprise a plurality of setting tools, each tool having an associated signal generating device and controlled by said system to be operated in a pre-determined sequence.
It is also preferable that the system will comprise an automated fastener feed system for supplying blind fasteners to the or each setting tool in a pre-determined sequence.
It is usual that the fastener setting tool will comprise a fluid actuated piston for applying load to the fastener whereby the signal generating device may comprise a pressure transducer for measuring the pressure applied to the piston as indicative of the load applied to the fastener. The applied load could, alternatively be determined by a number of alternative methods and associated devices including load cells, strain gauges or, more particularly, piezo-electric load measuring devices. The signal processor of the system may itself comprise a visual display for plotting the signal output versus time, either by way of a hard copy plot (such as a printer) or by a visual display or computer screen. The system may also comprise an indicator means, which could include the visual display discussed above, which indicator means being actuated in response to the output signal generated by the measuring method discussed above to indicate non compliance of the rivet setting procedure.